There exist in today's market devices, such as document scanners, which serve applications for which it is desirable to illuminate the scanned object, typically a document, with substantially uniform illumination. The scanned documents, for example ID cards, drivers' license, business cards and the like, are typically analyzed to extract information from the scanned document and non-uniform illumination degrades the ability to extract the desired information.
Illumination coming from external sources such as room illumination often creates uncontrolled reflections on the scanned document, thereby distorting the image of the scanned document. Internal light sources may also bring about problems of reflections that distort the uniformity of the illumination of the scanned document, thereby causing the image of the scanned document not to be a substantially true image of the scanned document.
When imaging a document with a camera, the document needs to be illuminated. Reference is made to FIGS. 1a (Prior art) and 1b (Prior art). FIG. 1a illustrates how hot spots 40 are formed by direct illumination of document 10. In conventional illumination methods, an example of which is shown in FIG. 1a, illumination positions 31 and 32 enable the return of the majority of light from document 10 to the lens of camera 50, but give rise to a problem known in the art as hot spot, where the light source (30) itself is imaged by camera 50. FIG. 1b depicts hot spot 40 caused by direct illumination of the scanned document 10 or by light entering the chamber from external source, for example thorough bare areas of the scanner glass window.
US application 20080285094 (US20080285094) by Iuval Hatzav et al, the disclosure of which is included herein by reference, provides a closed illumination chamber that yields uniform illumination of a document to be imaged. Referring to FIG. 2 (Prior art), a scanner with an illumination chamber 60, as provided by US20080285094, is shown. Light source 35 is disposed such that light source 35 is hidden from camera 50 and thereby cannot form a hot spot. But the device provided by US20080285094 is relatively large in size for a portable device.
Thus there is a need for and it would be advantageous to have a light mixing chamber for illuminating an object with substantially uniform illumination which is relatively small in size.
The terms “ray”, “light ray” and “light wave” are used herein interchangeably.
The terms “illumination chamber” and “light mixing chamber” are used herein interchangeably.
Angle of Incidence and Critical Angle
In geometric optics, the “angle of incidence” is the angle between a ray incident on a surface and the line perpendicular to the surface at the point of incidence, called the normal (N). The principle of Fresnel's equations, explains the reflection and the refraction of waves as the move from one medium, having one refractive index (n1) to another medium, having a second refractive index (n2). As a wave moves from one medium, having one refractive index to another medium, having a different refractive index, the point of incident acts as a new wave source, and one portion of the wave energy reflect back into the first medium and another portion penetrates through the second medium. Typically, glass has a refractive index n of ˜1.5 and air has a refractive index n of ˜1.0.
Further more, according to the principle of Huygens, when a ray hits a reflective surface, the point of incidence is in fact the center of a fresh disturbance and the source of a new train of light waves, and that the advancing wave as a whole may be regarded as the sum of all the secondary waves arising from points in the medium already traversed.
FIG. 3 (Prior art) illustrates light wave 20 moving through a medium having a refractive index n1, hitting another medium, having a refractive index n2 and then again, moving into the medium having a refractive index n1. Light wave 20 moves through the medium having a refractive index n1 and hits the medium having a refractive index n2 at an angle of incidence θi Part of the wave light energy reflects back at an angle θr, whereas θr=θi, forming light wave 22. The other part of light wave 20, light wave 24, moves through the medium having a refractive index n2, but angle θi changes to θt1, depending on the ratio between n1 and n2. The relation between θi and θt1 is given by the following equation (Snell's law):n1 sin(θi)=n1 sin(θt1)   (1)As light wave 24 moves through the medium having a refractive index n2 until wave 24 hits another medium layer having a refractive index n1. Light wave 24 splits to reflective portion 23 and to refractive portion 24: reflective portion 23 returns at an angle equal to θt1, and refractive portion 24 proceeds at an angle which changes from θt1 to θt2.
The energy distribution of the reflective portion 23 and to refractive portion 24, respectively, can be shown by Fresnel equations. When the light wave is at near-normal incidence to the interface (θi≈θt1≈0), the reflection coefficient R and transmission coefficient T are given by:
                    R        =                              (                                                            n                  1                                -                                  n                  2                                                                              n                  1                                -                                  n                  2                                                      )                    2                                    (        2        )                                T        =                              1            -            R                    =                                    4              ⁢                                                          ⁢                              n                1                            ⁢                              n                2                                                                    (                                                      n                    1                                    -                                      n                    2                                                  )                            2                                                          (        3        )            whereas for common glass, the reflection coefficient is about 4%.
It should be noted that in a partially transparent glass more rays are reflected back. The glass can be coated by materials having a refractive index n typically in the range of 2.7-5.0.